3x(3x+15)=90

Solution for 3x(3x+15)=90 equation:

3x(3x+15)=90

We move all terms to the left:

3x(3x+15)-(90)=0

We multiply parentheses

9x^2+45x-90=0

a = 9; b = 45; c = -90;
Δ = b2-4ac
Δ = 452-4·9·(-90)
Δ = 5265
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{5265}=\sqrt{81*65}=\sqrt{81}*\sqrt{65}=9\sqrt{65}$
$x_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(45)-9\sqrt{65}}{2*9}=\frac{-45-9\sqrt{65}}{18}
$
$x_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(45)+9\sqrt{65}}{2*9}=\frac{-45+9\sqrt{65}}{18}
$